1. Field of the Invention
The invention relates to workpiece bending method and apparatus for forming a final formed article that has two surface that are continuous across a boundary that is a bend portion, by bending a flat workpiece through the use of a stationary die and a movable die that are disposed facing each other.
2. Description of the Related Art
In recent years, the achievement of both collision safety improvement and a vehicle body weight reduction that is made for the purpose of cutting down the amount of CO2 emission through fuel economy improvement is a most crucial issue regarding motor vehicles. As a measure for meeting both of these conflicting requirements, the application of a high-strength steel sheet to a motor vehicle body is increasingly adopted. A press-formed steel sheet as a motor vehicle component member basically has a so-called box-like or hat-like sectional shape in which a plurality of inclined areas are formed. In order to secure dimensional accuracy of the press forming, it is important to control the amount of spring-back of the inclined areas. However, since a high-strength steel sheet having high material strength (e.g., a material strength of about 590 MPa) has great spring-back following the press forming process, it is difficult to secure good dimensional accuracy without some measure.
Therefore, in a common practice according to the related art, the amount of spring-back following the press forming is taken into account in the designing of the forming die so that the press-forming object workpiece is over-bent. In this manner, a dimensional accuracy of the bend angle can be secured.
However, the material strength of the high-strength steel sheets used as motor vehicle component members has become higher year by year. In recent years, it is highly necessary to apply high-strength steel sheets having material strength of 980 MPa or higher, to motor vehicle component members.
Thus, if the material strength of the high-strength steel sheet is heightened, the post-press-forming spring-back also increases. Therefore, in the case where a certain amount of spring-back is taken into account in the construction of the forming die as in the related art, the amount of spring-back taken into account becomes large so that the number of man-hours needed to repair or renew the forming dies increases and the repair operation becomes complicated. Besides, in some cases, depending on the shape to be press-formed, the over bending angle is less than 90°, which may cause a problem In the press forming. Furthermore, in the case where the material strength of the high-strength steel sheet is high, the characteristics, including the material strength and the like, vary to great extents from a workpiece to another; therefore, even if the spring-back is taken into account in the construction of the forming dies, the bend angles after the press forming are not consistent, and it is difficult to secure bend angle accuracy.
Besides, a press-forming method that reduces the amount of spring-back in the press forming of a high-strength steel sheet is shown in Japanese Patent Application Publication No. 2005-254262 (JP-A-2005-254262).
In the metal sheet processing method shown in Japanese Patent Application Publication No. 2005-254262 (JP-A-2005-254262), a vertical wall of a formed article that has a hat-like sectional shape is formed to about a half of the height thereof in a first forming process, and the rest of the height of the vertical wall is added and connected to the about half of the height of the wall in a second forming process. In the first and second forming processes, the bending and unbending at a die shoulder of a die is performed. In a disclosed method, the direction in which the steel sheet is pressed to the die shoulder in the second forming process is made opposite to the direction of the pressing to a die shoulder performed in the first forming process. Therefore, the wall formed in the second forming process serves as a spring-go element, and thus reduces the spring-back of the formed article.
However, the processing method described in Japanese Patent Application Publication No. 2005-254262 (JP-A-2005-254262) is intended to reduce the amount of spring-back in the vertical wall portion, and not to reduce the amount of spring-back of a bend portion of the workpiece. Therefore, the application merely of the forming method described in Japanese Patent Application Publication No. 2005-254262 (JP-A-2005-254262) will not secure dimensional accuracy of the bend angle.